Method and apparatus for thermal exchange with two-phase media

ABSTRACT

In a temperature control system using a controlled mix of high temperature pressurized gas and a cooled vapor/liquid flow of the same medium to cool a thermal load to a target temperature in a high energy environment, particular advantages are obtained in precision and efficiency by passing at least a substantial percentage of the cooled vapor/liquid flow through the thermal load directly, and thereafter mixing the output with a portion of the pressurized gas flow. This “post load mixing” approach increases the thermal transfer coefficient, improves control and facilities target temperature change. Ad added mixing between the cooled expanded flow and a lesser flow of pressurized gas also is used prior to the input to the thermal load. A further feature, termed a remote “Line Box”, enables transport of the separate flows of the two phase medium through a substantial spacing from pressurizing and condensing units without undesired liquefaction in the transport lines.

REFERENCES TO PRIOR APPLICATIONS

This application relies for priority on similarly titled applications ofthe same inventors, namely a first provisional application filed Sep.23, 2008, Ser. No. 61/192,881 and a second provisional application filedMay 20, 2009, Ser. No. 61/179,745.

BACKGROUND OF THE INVENTION

With the growth of modern technology, improved temperature controlsystems have also been sought for maintaining a thermal load at aprecise temperature under energy intensive conditions. Many such controlsystems also are required to change the temperature of the thermal loadin accordance with process conditions, sometimes with great rapidity. Asone illustration, semiconductor manufacturing equipment and processesare often dependent upon temperature control of the wafers or otherelements on which various surfaces are being deposited or etched, usingtechniques which are highly energy intensive. It is thus often necessaryto maintain a large semiconductor wafer which serves as the base forformation of thousands of minute complex integrated circuits, underprecise temperature control, as the wafer is processed, as under plasmabombardment. By such processes, minute patterns may be selectivelydeposited or etched in the wafer surface.

Semiconductor manufacture is referenced here merely as one example ofone process in which there is a need for precise temperature controlunder dynamic conditions. Other processes in which there are current orprospective demands for such capabilities will present themselves tothose skilled in the art.

In the past, temperature stability in the item being processed has oftenbeen achieved by using particular fluids and geometries to defineeffective heat sinks, for withdrawing or supplying thermal energy fromthe operating zone as needed, to establish a desired effectivetemperature level in the item. It has been common, heretofore, to employa thermal transfer medium which remains typically liquid throughout theentire temperature range used in a process. This medium can maintainadequate thermal transfer capability and at the same time avoid thecomplexity and unpredictability that would be introduced if a change ofphase from liquid to vapor were to be introduced, wholly or partially.

Although the state of the art has been constantly evolving, fewdistinctly different methods were employed until a novel thermal controltechnique was introduced by Kenneth W. Cowans et al employing energytransfer using different phases of the same medium. Patents entitled“Thermal Control System and Method” (U.S. Pat. Nos. 7,178,353 and7,425,835) have issued on this concept and are assigned to the assigneeof the present application. This concept employs the thermodynamicproperties of a refrigerant in both vapor and liquid phases, properlyinterrelated to exchange thermal energy with a load so as to maintainthe temperature at a selected target level within a wide dynamic range.Consequently, the refrigerant can heat or cool a product and process,such as a semiconductor wafer of large size, at a single or a successionof different target temperatures. This concept has been referred to forconvenience by the concise expression “Transfer Direct of SaturatedFluid”, abbreviated TDSF. This descriptor recognizes and in a sensesummarizes the operative sequence, in which a medium is first compressedto a high temperature gaseous state, then divided, under control, intotwo interdependent flows. One flow path maintains the fluid in highpressure gaseous phase, but in this flow path the flow rate and mass arevaried in accordance with the target temperature to be maintained.Variation of the one flow affects the differential flow in the otherpath, in which the refrigerant is converted, by cooling, to liquid phaseand the flow is then further cooled by expansion. In this path the flowrate is dependent on the heat load presented to the system. Typically,the flow in this liquefied path is regulated by a standard refrigerationthermo-expansion valve (TXV).

As disclosed in the referenced patents, the two flows, of high pressuregas and cooled expanded fluid/vapor, are recombined in a mixer beforedelivery to the thermal load. The target temperature for the load isestablished by adjusting the balance between the two flows by admittinga selected amount of hot gas flow, controlled such that needed pressure,temperature and enthalpy are maintained in a continuous loop.

The TDSF concept has numerous advantages. Some can be best expressed interms of the range of temperatures that can be encompassed from hot(entirely pressurized gas) to maximum cooling (entirely expanded vapor).The concept also enables the load temperature to be maintained withprecision. The target temperature can be adjusted bi-directionally andrapidly.

The use of a refrigerant having a temperature/pressure transition thatis somewhere in mid-range relative to the operating temperature band,however, creates possibilities for undesired changes in refrigerantstate under certain operating conditions. Situations have beenencountered in which performance limitations have been imposed on TDSFsystems because of installations which introduce substantial pressuredrops or long transport lines for the refrigerant. These conditions canarise because, in a two-phase medium, pressure drops are alsoaccompanied by temperature variations. For example, long line lengthsfrom compressor and condenser units to a semiconductor processing sitemay be required for operative or geometrical considerations. Heretofore,installations which have inherently required the use of long transportdistances for refrigerant media have sometimes imposed restraints on theuse of the TDSF concept or the use of special expedients which addundesirable complexity and cost. It is also true that long lines canintroduce another complication, that of ‘puddling’: If this occurs, theliquid phase can separate from the two-phase mixture creating variationsin mass flow at the line's end. This can adversely alter controlcharacteristics due to surging conditions as pure liquid and puregaseous phases alternate with mixed two phase flow.

SUMMARY OF THE INVENTION

The present invention discloses a novel implementation of the TDSFconcept of separating and later recombining a high pressure gas phase ofa two-phase refrigerant medium with a cooled, liquefied and thenexpanded differential flow of the same medium, and application of themedium to the thermal load. In accordance with the invention theprincipal phase of the refrigerant that is propagated through thethermal load while the load is being heated is the cooled expandeddifferential flow. The combination of cooled expanded flow through thethermal load with the modulated high pressure gas flow occurs after aswell as before the thermal load, so that this approach has been termed“Post Load Mixing” (PLM). The media fed into the thermal load heatexchanger is stabilized in temperature throughout its flow through thatexchanger because it is responsive both to the enthalpy of the expandedcomponent and the pressure modulated by the hot gas in the mixingprocess.

The PLM approach uses the two different phase states of the refrigerantin a uniquely integrated manner. The pressure of the suction line to thecompressor is influenced by the mass of refrigerant received, since thecompressor is a device that processes a fixed volume per unit of time.In the PLM system the flow through the thermal load has a smallerdifferential in temperature than would exist with unidirectionaltransport of fully mixed dual flows, and the thermal load temperaturecan be thus more tightly controlled. Essentially, the flow through thethermal load is so controlled as to be mainly or completely the cooledexpanded component, and in consequence the pressure drop undergone bythe refrigerant in passing through the load is lessened. Furthermore, bypost load mixing after the refrigerant has passed through the load, therefrigerant passing through the thermal load has a greater percentage ofliquid than if all the hot gas had been mixed before the load and thushas a higher heat transfer coefficient, so that thermal exchange is moreefficient, particularly at and near the last portions of the heatexchanger passage.

The PLM concept employs some mixing of the two flows both before andafter the thermal load, but in a selectable proportionality. This isdone in a preferred embodiment by including two impedances in the pathssupplying the high pressure hot gas to the mixing tees. Said impedancesare settable as to magnitude. A flow of high pressure gas is branchedoff and combined with the cooled expanded flow at an input mixer coupledto the input to the thermal load. The flow bypassing the thermal load isalso directed through a series-coupled solenoid valve which can becontrolled so as to enable rapid changes of operating mode between postload mixing and fast heating of the thermal load. Said solenoid valve isclosed when rapid heating of the thermal load is desired. This isusually employed when switching the load from one temperature to ahotter temperature, as when a chuck that is normally cold duringprocessing is removed from the system to allow repair to beaccomplished. Rapid heating will thus minimize the time needed for suchrepair and changeover.

The post load mixing approach may be used in certain geometries orapplications requiring that the refrigerant be transported over arelatively large distance between the energizing (compressing andcondensing) sites and the sites at which thermal exchange occurs. Inaccordance with the invention, substantial advantages are achieved inthese situations by deploying the principal flow adjusting, combiningand mixing circuits in a geometrically compact and thermodynamicallyadapted post load mixing unit, denoted the PLM line box (LB).

The PLM LB is for disposition in proximity to the thermal load andincorporates conduits for high pressure gas flow, liquefied refrigerantlow, and return flow, as well as a thermo-expansion valve (TXV), anequalizer for the TXV, and check valves and mixing tees. Theconfiguration, which forestalls mixing before the transport lines, isrealized within a volume that is about one cubic foot or less. This unitmay be described as comprising a remote control box.

In this combination, the thermo-expansion valve is proximately coupledto a temperature sensing bulb responsive to the temperature in thereturn line from the load after the mixing tee located downstream fromthe thermal load. Said thermo-expansion valve is also coupled with apressure sensing line to the return line in a position proximate saidtemperature sensing bulb, which coupling serves to establish theexternal equalizer function. In those installations displaying a minimalpressure drop through the thermal load said thermo-expansion valve canbe of the internally equalized type. When such non-equalizing valves areemployed said coupling to the return line is not used. The two mixingtees are disposed separately, one before and one after the thermal load.The system may include a check valve before the first mixing tee, and,for flow regulation, a flow orifice is disposed before each mixing tee.A solenoid valve is located in series with the second mixing tee.Consequently, despite the fact that long transport lines may be neededbetween the phase conversion, energy demanding portions of the systemand the thermal load at the process site, needed phase conversions andflow modulations are effected reliably without the danger ofaccumulation of internal liquids.

In accordance with other features of the invention, where transportlines and conditions present only marginal probability of liquefaction,the transport lines from the proportional valve and the thermo-expansionvalve can be disposed to parallel but insulated externally from eachother before being coupled to a mixer in the PLM configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention may be had by reference to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of a system for thermal exchange usingtwo-phase media in accordance with the PLM invention;

FIG. 2 is a block diagram representation of a PLM system incorporating acompact remote control box;

FIG. 3 is a perspective view, in plan, of an example of the elementsinterior to a remote control box

FIG. 4 is a fragmentary view of a portion of an alternate arrangementfor transporting different phases of a refrigerant, processed inaccordance with the TDSF concept, prior to mixing;

FIG. 5 is a Mollier diagram evidencing thermodynamic changes in statesexisting in a typical system in accordance with the invention, such asshown in FIG. 1, and:

FIG. 6 is a chart of tested performance characteristics of a system inaccordance with the invention, in comparison to the performance of aconventional temperature control system, referred to as a “conventionalchiller”.

DETAILED DESCRIPTION OF THE INVENTION

A generalized system utilizing post load mixing (PLM) is shown in FIG.1, to which reference is now made. The thermal control system 10 or“TCU” is consistent with the TDSF concept but differentiated byincorporating the PLM approach, and forms a closed loop that encompassesan active thermal control system (TCU) 10 and a thermal load 30. Thethermal load 30 is typically a heat exchanger that functions with aprocessing unit (not shown), such as a chuck for processingsemiconductors. In the thermal control system 10 a refrigerantcomprising a medium such as R-507 is input to a compressor 12 in gaseousform and a pressurized output is provided therefrom into a main line 13.One branch from the main line 13 includes an air cooled (in thisexample) condenser 14 having an external air-cooled fin structure 15engaged by flow from a fan 16 shown only symbolically. The condenser 14provides a fully or substantially liquefied output of refrigerant at anessentially ambient temperature in a first output path 20.

A separate branch from the compressor 12 output 13 is taken from ajunction before the condenser 14 to direct pressurized hot gas from thecompressor 12 into a second flow path 22. This second flow path 22includes a proportional valve 24 that is operated by a controller 18 soas to adjust the proportion (in mass flow rate) or hot gas that is to beused out of the compressor 12 output. This adjustment modulates the twoflows and ultimately determines the proportion of hot gas to be employedin the consequent mixture of the two flows, as described below. Theadjustment consequently sets the target temperature for the thermal load30.

In the first branch 20 the output from the condenser 14 is applied to athermo-expansion valve TXV 26, this output being dependent on anddetermined by the differential temperature between the superheated gasas sensed a proximate by bulb 35 and the temperature of output fluidfrom the second mixer 32 a point in line 51 adjacent where the bulb 35is located. The thermo-expansion valve 26 thus senses the pressuredifference between liquid contained within bulb 35 and the pressuresensed by a line 48 connected to externally equalized TXV 26. The outputflow from the TXV 26 is here coupled to the thermal load 30, which isdepicted only generally. Said output flow from the TXV 26 travelsthrough a delta P valve 49 which valve performs the same function asdisclosed in U.S. Pat. No. 7,178,353. After passing through valve 49 theexpanded cooled output from the TXV 26 mixes with some of the hot gas inthe first mixing tee 50. The output 31 from the load 30 is, inaccordance with the PLM approach, returned to the input of thecompressor 12 via one input of a second mixing tee 32, which alsoreceives, at a separate input, some of the output from the proportionalvalve 24. The output line from the second mixing tee 32 returns to thecompressor 12, but the input pressure of this return flow is sensed onroute to the compressor 12 input by the external equalization bulb 35which is coupled into the TXV 26 via the line 36. This connection alsoprovides the known external equalization feature disclosed in thepatents referred to above and in other patents and applications on theTDSF system, so that it need not be described in further detail. Inaddition, the controller 18 for the proportional valve 24 receives atemperature input from a sensor 38 that is responsive to the temperaturelevel at the thermal load 30. Alternatively, said temperature sensor 38may be mounted so as to sense any other location that is desired toregulate.

The PLM dual flow, dual mixing system, has other features andadvantages. A solenoid valve, labeled SXV 54 is in the path from theproportional valve 24 to the second mixer 32. The SXV 54 is controlledby the controller 18, so it can be shut off whenever the system isprogrammed to make a change in the target temperature from one level toa higher level. Shutting off this path at the SXV 54 assures that allhot gases flow to the input of the first mixer 50, and more rapidlyincrease the temperature of the flow into the thermal load 30. In theinput to the SXV 54, a settable impedance, shown symbolically,constituting a controllable orifice 78 is included, in parallel to acomparable settable impedance or controllable orifice 79 in the directpath to the first mixer 50. By the use of these control orifices 78 and79, the two separate flows of pressurized gas fed into the first mixer50 and second mixer 32 can be proportioned and balanced as desired. Thesystem also includes, as shown, a heater 117 in the input to thecompressor 12, which heater 117 may be activated by the controller 18 toconvert a liquid containing mixture returning from the second mixer 32to the wholly gaseous phase for proper operation of the compressor 12.

Mixing the hot gas from the proportional valve 24 with the cooledexpanded flow from the TXV 26 after the thermal load 30 retains theessential benefits of the TDSF system, but offers particular addedbenefits. These are particularly applicable where substantial pressuredrops or differentials in heat transfer coefficients may be encounteredor exist within thermal load 30. The mass flow from the proportionalvalve 24, when combined with the system flow at the second mixing tee 32and also with the TXV 26 output to the first mixing tee 50, modulatesthe pressure within the load 30. This variation affects the temperaturewithin the circuit and thereby controls the temperature of the load.With PLM, the temperature level across a thermal load, such as asemiconductor chuck can be contained within tolerances that are moreprecise than previously expected. Tests of a practical system show areduction in temperature differential to 3° C. from a prior 10° C.differential.

The media fed into the thermal load 30 is stabilized in temperaturethroughout its flow path in the heat exchanger therein because of thetotal pressure of the refrigerant fluid, which pressure is controlled bythe proportion of hot gas propagated into the circuit. The pressure ofthe refrigerant in the suction line to the compressor 12 is influencedby the mass passed into the compressor, which compressor 12 processes afixed volume per unit of time. Because of these interrelated factors,the thermal load 30 is more tightly temperature controlled than innon-PLM based systems. In the system shown, the flow through the thermalload 30 is generally restricted so as to be completely or almostcompletely that refrigerant that flows through the thermo-expansionvalve 26. By so limiting the flow, the pressure drop undergone by therefrigerant passing through the load is lessened. Also, since the hotgas is mixed at the second mixer 32 with the two-phase output of the TXV26 after the output has passed through the load 30 there is a greaterpercentage of liquid in the mix at this point. Thus the heat transfercoefficient is maintained high throughout the thermal load 30.Therefore, adjustments in the two flows can also be made after sensingthe thermal load temperature, in order to anticipate temperaturedifferentials.

Reference should now be made to the Mollier diagram of FIG. 5 whichdepicts the thermodynamic variations in enthalpy (abscissa) vs. pressure(ordinate) in a complete cycle for the system of FIG. 1. Thepressure-enthalpy points in FIG. 5 are identified by numbers inparentheses to correspond to the similarly identified numbers inbrackets positioned around the block diagram of FIG. 1. Thus the inputat point (1) to the compressor 12 is, as seen in FIG. 5 increased by thecompressor in pressure and enthalpy to point (2) before some of it isliquefied in condenser 14 to point (3). After controlled expansion topoint (4) in the TXV 26, then consequently mixing some hot gas from theproportional valve 24 at point (6) in the first mixer 50 (see alsoFIG. 1) results in an increase in enthalpy to point (4 a). Thisinterchange is illustrated in FIG. 5 by the dotted line 57 betweenpoints (6) and (4). Passage of the refrigerant through the thermal load30 absorbs heat from thermal load 30 and shifts the enthalpy to thepoint (5). The injection of pressurized hot gas at the input to thesecond mixing tee 32 of FIG. 1, as also shown at point (6), and depictedby dotted line 58 on FIG. 5. This input adjusts the heat and enthalpyfrom point (5) to point (1). The addition of hot gas at the mixing tees50 and 32 also adjusts the pressure of the throughput flow, thus furtherand more precisely adjusting the temperature of the refrigerant at thethermal load 30. Consequently, the controller 18 may set theproportional valve 24 to vary the hot gas mass flow, and responsively,the cooled expanded flow from the TXV 26, to create pressure andenthalpy parameters at the operative levels needed to achieve a targettemperature at the thermal load 30. In this system, the restriction ofthe direct flow through the load 30 reduces the pressure drop throughthe load 30 to a minimum. Also, the heat transfer coefficient within theload 30 is maintained at a maximum. Accordingly, the system providessuperior results in achieving and maintaining target temperature.

This conclusion is exemplified by factual results achieved in the use ofthe PLM concept in controlling the temperature of an electrostatic chuckused in semiconductor processing. In prior systems, temperature controlunits have used a liquid mix of thermal exchange fluid, and providedtemperature differentials of the fluid through the chuck typicallyaveraging 10° C. (±5° C.). Using post load mixing, however, thetemperature differential through the entire area of the chuck wasreduced to no more than about ±3° C.

FIGS. 2 and 3 disclose an alternative which resolves problems ofunwanted liquefaction in transporting a two-phase medium in a long linesystem employing the TDSF concept. For completeness, the system diagramof FIG. 2 partially repeats the principal elements of FIG. 1, placingthe principal subsystems that provide phase conversion or energyconsumption in a single block labeled “TDSF system” 10. From thissystem, a hot gas line 63 controlled by a proportional valve 245, acooled liquid flow line 64 from the condenser 14 and a return line 65 tothe compressor 12 are all coupled to a remote control box here termed aPLM Line Box (or LB) 70. The energy converting units in the TDSF system10 are not attempted to be depicted to scale, in the interest of clarityand understanding, since the Line Box 70 is exaggerated, as thesubsystems of interest. The system of FIG. 2 solves a problem which mayarise because of the manipulation, in the TDSF system, of gas and liquidphases of refrigerant, in an advantageous manner for temperaturecontrol. Concurrent modulation can introduce undesired liquefaction asin the transport of the two-phase medium along a long path. The systemof FIG. 2 addresses this problem effectively, and details of a specificimplementation further confirming this result are shown in FIG. 3, towhich reference should also be made.

In order efficiently to utilize the thermal and fluid pressure energy inthe lines 63 and 64 in propagating fluids to and from the physicallywell separated TDSF system 10, the operative elements for mixing andcontrol are principally located relatively remotely in what is herecalled a “PLM Line Box” 70, as shown in both FIGS. 2 and 3. In thispractical example, the Line Box 70 is very small in volume by comparisonto the energy generating subsystems. The example shown in FIG. 3 is12″×12″×6″, or 864 in³, and it is typically located within about 1 meteror less from the thermal load 30 input and output points. In the LB 70,the condensate line 64 is directed to a thermo-expansion valve 26 theoutput of which is applied to a Δp valve 76 for pressure reduction, asis well known in TDSF systems. The thermo-expansion valve (TXV) 26 isexternally equalized by pressure transmitted from a point in return line65 via line 36. Consistent with the system diagram of FIG. 1, at asuitable point in line 65 a sensor bulb 35 is disposed in thermalcommunication with the return line 65 to sense the temperature of flowreturning to the TDSF system 10. The output from the Δp valve 49 iscombined with a portion of the high pressure hot gas flow from the line63 that is transmitted through a check valve 52 to one input of a firstmixer 50, which also receives a separate input from the Δp valve 76. Theoutput from the first mixer 50 is, as is disclosed above in relation toFIG. 1, applied to the input of the thermal load 30.

Also consistent with the arrangement of FIG. 1, the output of thethermal load 30 is coupled to one input of a second mixer 32 having asecond input ultimately receiving the flow of pressurized hot gas fromthe line 63. This bypass flow is, consistent with FIG. 1, directedthrough a solenoid valve, (designated SXV) 54 that is operated bysignals from the controller 18. The input to the SXV 54 is applied viathe flow control orifice 78, inserted to balance flows between thebypass path and the separate path to the thermal load 30. From the flowbalancing or control orifice 78 the flow is directed to the second inputof the second mixer 32 that is in circuit with the return line 65 to thecompressor 12 input.

The arrangement of elements inside the PLM Remote Box 70 is shown threedimensionally in FIG. 3, with the depicted elements being numberscorrespondingly to the elements in FIG. 2. Although the volumetric size,as set forth above, is very compact by comparison to the compressor andcondenser units, it is fully functional for the semiconductor chuckinstallation. The unit can be further compacted as desired.

Incorporating the operative control elements for unification and mixingof the two flows of refrigerant in the very small volume illustrated inFIGS. 2 and 3 resolves the problem of unwanted temperature variationsand accumulation of liquid in the return line, all while retaining thebenefits of the PLM approach. The PLM flow balance orifices 78 and 79control the flow proportions both before and after the thermal load 30.Furthermore, the added line in the TDSF system 10 provided by the PLMRemote Box 70 directs the bulk of hot gas around the load so that it canunite with the two-phase liquid after the load 30. Consequently the“quality” of the fluid that is fed to control the thermal load 30 islowered, while still operating in the PLM mode. In effect, there is anincrease in the liquid content in the two-phase mixture that is suppliedto the load, which enhances the cooling efficacy of the two-phaseliquid. The advantages of employing the PLM mode in conjunction withlong line installations, are made evident in an objective way by thecomparison of performance characteristics in FIG. 6, to which referenceis now made. This comparison is between a conventional chiller, such asan Advanced Thermal Sciences, MP40C, and a “direct chiller” of the TDSFtype that incorporates the present post-load mixing Long Lineimprovement. In all individual parameters that are significant tothroughput and uniformity the chiller disclosure herein confirms thesignificant improvement in performance over a commerciallystate-of-the-art unit. Care was taken to ensure test conditions werecomparable in all respects.

As a qualitatively limited alternative, when substantial line lengthsmight introduce problems with liquid puddling within transport lines,unstable temperature changes due to puddling can be limited or avoidedusing the insulation technique depicted in FIG. 4. The supply line 22for cooled expanded flow and the output line 25 from the proportionalvalve 24 (both as shown in FIGS. 1 and 2) are insulated from each otherwithin a jacket 66 until they reach the near vicinity of the load 30, asat the mixer 50.

Although there have been described above and illustrated in the drawingsvarious forms and expedients for post load mixing, the invention is notlimited thereto but incorporates all features and alternatives withinthe coverage of the appended claims.

1. A system for utilizing separate flows of high pressure, hightemperature fluid from a compressor to control the temperature of athermal load having an input and an output, and comprising: a firstpropagation path receiving high pressure, high temperature flow of thefluid in gaseous state from said compressor and comprising acontrollable flow valve providing a variable mass flow rate, said firstpropagation path including circuits coupling said variable flow to theoutput of said thermal load; a second propagation path receiving hightemperature, high pressure fluid from said compressor, at a ratedependent on the mass flow extracted in said first propagation path; acooling flow circuit including a condenser disposed in said secondpropagation path for liquefying the received pressurized gas, and saidcooling flow circuit further including a thermo-expansion valve forexpanding and thereby cooling said second flow to an at least partiallyvapor state at an output; and input circuits coupling the output of saidcooling flow circuit to the input of said thermal load; and a mixerhaving first and second inputs and coupled to receive the flow from saidfirst propagation path at a first input and the output flow from saidthermal load at a second input, said mixer having an output providing amixed output flow to the compressor, such that the temperature of saidthermal load is controlled by the mixed flow.
 2. A system as set forthin claim 1 above, further including a controller selecting a targettemperature for said thermal load and coupled to adjust saidcontrollable flow valve in response thereto, and a temperature sensorresponsive to the temperature of said thermal load and providing atemperature reading from the thermal load to said controller.
 3. Asystem as set forth in claim 1 above, further including a first mixerhaving first and second inputs coupled to the output of saidcontrollable flow valve and the output of said cooling flow circuitrespectively, and an output coupled to the input of said thermal load,and the system also including a second mixer having a first inputcoupled to the output of said thermal load, a second input coupled tosaid first propagation path, and an output coupled to include thecompressor input to define a temperature control loop for the thermalload.
 4. A system as set forth in claim 3 above, wherein the couplingsfrom said first propagation path to said first and second mixerscomprise at least one settable impedance for balancing the proportion offlows between the first and second propagation paths.
 5. A system as setforth in claim 4 above, where said at least one settable impedancecomprise two flow balancing orifices each coupling flow in the firstpropagation flow path to a different one of the first and second mixers,and wherein the first path from said compressor to the output of thethermal load includes a controller responsive shutoff valve.
 6. In athermal control system using flows of a two-phase refrigerant mediadivided from an initial high pressure gaseous phase into both a hightemperature, high pressure gaseous phase and a post-condensed expandedphase flowing in a recirculating loop to provide thermal control of aload. Said load having an input and output and said system usingvariable mixes of the two phases in controlled proportions, theimprovement comprising an intermediate input circuit coupling therefrigerant media in said high temperature, high pressure phase to theoutput of said load, and the post-condensed expanded phase to the inputof the load, to adjust the temperature level in the recirculating loopby changing the total mass flow of said refrigerant media in the highpressure gaseous phase, while concurrently tending to maximize theliquid content in the post-condensed expanded phase.
 7. A thermalcontrol system as set forth in claim 6 above, wherein said intermediateinput circuit comprises a serially connected shutoff valve and asettable impedance in circuit with the flow path of the hightemperature, high pressure gaseous phase, and a second settableimpedance in circuit with the flow path of the post-condensed gaseousphase.
 8. In a thermal control system employing a two-phase refrigerantmedia in both a high temperature, high pressure gaseous phase and adifferential flow of post-condensed expanded cooled phase to control thetemperature of a thermal load by combining controlled proportions of thetwo phases, the improvement comprising a first circuit including a firstmixer for combining said expanded cooled phase with a controlledfraction of said high temperature, high pressure phase and flowing thecombined flow substantially directly through said thermal load, and asecond circuit including a second mixer combining the major flow, lessthe controlled fraction of said high temperature, high pressure phasewith the thermal load output to intensify the liquid component level inthe flow through the thermal load to augment temperature control byreducing temperature and pressure differences in thermal exchange withthe thermal load.
 9. A thermal control system as set forth in claim 8above, wherein said first circuit and said second circuit each comprisea settable impedance for controlling the proportions of flows betweenthe two paths, wherein the mixed flow through the thermal load can beadjusted to maximize the liquid content and thermal transfer efficiency,and lower the pressure drop through the thermal load.
 10. In atemperature control system employing a two-phase thermal transfer mediumand mixing controlled proportions of said medium in a pressurized hotgas phase with an expanded liquid/vapor phase, to establish a targettemperature in a thermal load having an input and output, a combinationfor improving the resolution attainable in the target temperature,comprising: a compressor-condenser pressurizing-condensing loop forrecycling a primary flow of two-phase refrigerant said loop dividingpressurized gaseous flow into separate flows of pressurized hot gas andcondensed refrigerant in separate first and second paths, respectively,in the recycling loop; a variable control system including a controllerresponsive to a target temperature and including a mass flow controldevice in the first path for varying the flow in said first path in asense needed to attain the target temperature; an expansion device insaid second path for converting the differential of flow from theprimary flow refrigerant in the state of condensed refrigerant to anoutput of at least partially gaseous phase at lower temperature; a firstmixing device having two inputs and an output, a first input beingcoupled to receive the output of said expansion device in the secondpath, and the second input being coupled to receive a portion of thevariable flow from said first path, the first mixing device output beingcoupled to the input to said thermal load, and a second mixing devicehaving two inputs and an output, a first input being coupled to theoutput of said thermal load, and a second input being coupled to receivethe hot gas flow from the first path, and the second mixing devicehaving an output coupled to said pressurizing-condensing loop forrecycling the medium.
 11. A system as set forth in claim 10 above, wherethe first path includes a first flow control orifice before the coupledinput to the first mixing device, and a second flow control orifice andshutoff valve from the first path to the coupled input to the secondmixing device, the shutoff valve being responsive to the flow controlsystem, wherein the system further comprises a pressure dropping valvein the second path between the expansion device and the first mixingdevice, and a sensing device responsive to the temperature of the outputflows from the second mixing device and coupled to the expansion devicefor internal equalization thereof.
 12. In a system which generates, froman initial two phase cooling medium, divided separate flows of highpressure gaseous medium, and a differential flow of the medium in liquidform and the system requires thermal control of a thermal load at anoperative site that is physically displaced from the location at whichthe separate flows are generated and divided, a temperature control unitfor disposition at the operative site in proximity to the thermal loadcomprising: a thermo-expansion device receiving the liquid medium andproviding a cooled expanded flow of the medium proximate the thermalload; a first mixer receiving the high pressure gaseous medium at oneinput and providing an output to the thermal load, the first mixerhaving a second input receiving the pressurized gaseous medium; and asecond mixer having two inputs and receiving the pressurized gaseousmedium at one input and the output from the operative site, andproviding a return output to the system.
 13. A combination as set forthin claim 12 above, wherein the path providing high pressure gaseousmedium to the first mixer includes a flow balancing orifice and a checkvalve in series, and wherein the flow path to the second mixer includesa shutoff valve and a flow balancing orifice in series, and wherein thecombination includes a compact line by structure encompassing thethermo-expansion device, the first and second mixers, the flow balancingorifices, check valve and shutoff valve within its volume.
 14. In asystem for controlling the temperature of a thermal load by directtransfer of thermal energy with a two-phase refrigerant, wherein therefrigerant is separated into a controllable first flow of pressurizedhot gas and a second differential flow of condensed refrigerant, whereinthe thermal energy interchange is to be effected with said thermal load,and wherein said first and second flows of the refrigerant are generatedat a site which is substantially separated physically from the thermalload, such that a phase change introducing liquefaction might occur intransport of said refrigerant to said thermal load, a combination forforestalling undesired separation of liquid refrigerant in transport ofsaid first and second flows to said thermal load comprising: aconversion and control unit of substantially less than 1 ft³ in volumewithin less than about 2 meter (76″) from the thermal load, said unitincluding transport lines coupled from the system delivering said flowsof pressurized hot gas refrigerant and condensed refrigerant, said unitfurther comprising within its volume a thermo-expansion valve receivingsaid condensed refrigerant, a first mixer receiving the output of saidthermo-expansion valve and said flow of pressurized hot gas and havingan output coupled to said thermal load, and a second mixer receiving aportion of said hot pressurized gas flow at a first input and the outputfrom said thermal load at a second input.
 15. A combination as set forthin claim 14 above, wherein said conversion and control unit furthercomprises a bulb sensor responsive to the temperature of the output fromsaid second mixer and in fluid communication with said thermo-expansionvalve to provide external equalization thereof, a pressure droppingvalve in the line between the outlet of said thermo-expansion valve andsaid first mixer, and wherein the coupling transferring pressurized hotgas flow to one input of said second mixer includes a solenoid valve anda flow balancing orifice, and the line coupling pressurized hot gas flowto said first mixer includes a check valve.
 16. A method for atemperature control system using a two-phase refrigerant for direct heattransfer with a thermal load, wherein the refrigerant is divided intoflows of pressurized hot gas and also cooled expanded refrigerant aftercondensation, and the flows are transported to said thermal load throughphysical distances in which phase change in said refrigerant mightintroduce unwanted liquefaction in the lengths of transport line betweensaid thermal load and the sources of the flows, and wherein said dividedrefrigerant flows are at least partially mixed in both pre-load andpost-load positions relative to said thermal load, the method comprisingthe steps of: at least partially expanding said condensed refrigerant tolower the temperature thereof in close juxtaposition to the thermalload; premixing, in close juxtaposition to the thermal load, the atleast partially expanded refrigerant with a variable controlled flow ofa first portion of said pressurized hot gas; passing the premixed flowinto heat exchange relation with said thermal load; post-mixing thesecond, remaining portion of said flow of pressurized hot gas with theoutput premixed flow after said thermal load and in close juxtapositionto the thermal load, and recycling the post-mixed flow to saidtemperature control system to reestablish said pressurized hot gas flow.17. The method as set forth in claim 16 above, wherein the step of atleast partially expanding said condensed refrigerant includes the stepof externally equalizing the expansion in response to the temperaturelevel in said post-mixed flow, and wherein the steps further includemaintaining the temperature level at the sensing position at assuperheat level such that effective thermo-expansion is maintained. 18.A method of controlling the pressure and enthalpy relationships of atwo-phase media when combining a vapor/liquid post-component of acondensed phase of a media with a high temperature gaseous phase of thesame media, to meet target thermal levels desired to be maintained in aload having an input and output and being subject to temperature andheat transfer variations due to pressure deviations and heat transfercoefficient differentials arising from changes in media phase,comprising the steps of: applying a controlled proportion of thevapor/liquid post-component to the thermal load input and applying asubstantial proportion of the compressed component to the thermal loadoutput to raise enthalpy of the media in the thermal load.
 19. A methodof combining the pressure and enthalpy of a combined gaseous componentof a two phase media with a vapor/liquid post-component of a condensedphase of the same media, to meet target temperature levels desired to bemaintained in a load having an input and output and being subject totemperature and heat transfer variations due to pressure deviations andheat transfer coefficient differentials due to media phase, comprisingthe steps of: applying a controlled proportion of the vapor/liquidpost-component to the thermal load input and applying a substantialproportion of the compressed component to the thermal load output toraise the enthalpy of the media in the thermal load.
 20. A method ofcontrolling the temperature of a thermal load using a pressurizedtwo-phase fluid from a pressurizing energy source, in which method aninitial flow of said fluid is divided into a pressurized hightemperature gaseous flow and a cooled and then condensed and expandedflow of differentially higher liquid content than the initial flow, themethod comprising the steps of: varying the flow rate of said hightemperature gaseous flow, thereby differentially varying the flow rateof said cooled expanded fluid; passing at least a substantial portion ofsaid cooled expanded flow through said thermal load in thermal exchangerelationship with said load while maintaining a liquid content therein;mixing the output from said thermal load with said high pressure hightemperature flow, and returning the combined flow after mixing forrecompression by said pressurizing energy source and completion of acontinuous cycle.
 21. The method as set forth in claim 20 above,including the further steps of also mixing a portion of the hightemperature gaseous flow with the cooled expanded flow before passingthe mixture of both flows in thermal exchange relationship with saidload, whereby the load input receives cooled expanded flow that isthermally modified.
 22. The method as set forth in claim 21 above,further including the steps of separately adjusting the relative flowrates of the high temperature gaseous flow rate before mixing with thecooled expanded flow and before mixing with the output from the load.23. The method as set forth in claim 20 above, wherein said cooledexpanded flow is a liquid/vapor mix at a substantially lowertemperature, than one high temperature gaseous flow and wherein thetemperature of said thermal load is controlled primarily by modulationof said pressurized high penetration gaseous flow rate and the methodfurther includes the steps of sensing the temperature of said thermalload and modulating said gaseous flow rate in response thereto.
 24. Themethod as set forth in claim 20 above, wherein the steps of pressurizingsaid gaseous flow and varying said high temperature gaseous flow rateare effected at a relatively substantial distance from the thermal load,and wherein the steps of mixing said high pressure flow with said outputof said thermal load and expanding said condensed fluid are effected insubstantial proximity to the thermal load, whereby said two-phase fluidis transported said substantial distance without significantliquefaction before reaching the region of substantial proximity to saidthermal load.
 25. The method as set forth in claim 24 above, wherein thesteps of mixing both before and after the thermal load includeseparately adjusting the high temperature flows to achieve flow balance.26. The method as set forth in claim 20 above, further including thesteps of pre-mixing a proportion of said pressurized gaseous flow withsaid cooled expanded flow as an input for said thermal load, and furtherpost-mixing a proportion of said pressurized gaseous flow with themixture flowing from said output of said thermal load, and returning thepost-mixed flow for recompression by said pressurizing energy source.27. The method as set forth in claim 20 above, wherein the methodfurther includes the steps of adjusting said flow of the proportion ofthe pressurized gaseous flow after mixing with said output of saidthermal load, and further including the steps of selectively terminatingsaid high temperature gaseous flow for mixing after the load output tofacilitate rapid cooling of the load.